In a wheel hydraulic excavator having a variable displacement traveling motor in the related art, the motor displacement is controlled by driving a motor regulator in correspondence to the drive pressure at the traveling motor. In such a wheel hydraulic excavator, the motor displacement increases as the drive pressure rises to drive the motor at low speed with high torque and the motor displacement decreases as the drive pressure becomes lower to drive the motor at high speed with low torque.
More specifically, the motor displacement is fixed at a constant value (e.g., the minimum displacement) over a predetermined low motor drive pressure range so as to minimize the extent to which the traveling speed changes due to fluctuations in the motor drive pressure when traveling on flat ground or downhill, whereas the motor displacement is increased as the motor drive pressure increases beyond the predetermined range so as to increase the rotation torque at the motor during acceleration or when the vehicle is traveling uphill.
As either the front side (toward the toes) or the rear side (toward the heel) of the accelerator pedal in a wheel hydraulic excavator is depressed, the accelerator pedal is allowed to rotate along the forward/rearward direction. As the front side or the rear side of the accelerator pedal is depressed, the control valve is switched from the neutral position to a forward travel position or a reverse travel position, and pressure oil is supplied from a hydraulic pump to the traveling motor to generate a motor drive pressure. As the accelerator pedal is released while traveling, the control valve is switched to the neutral position, thereby cutting off the pressure oil supply from the hydraulic pump to the traveling motor. Subsequently, the vehicle travels with an inertial force and the function of the traveling motor is switched from the motor function to a pump function. The motor drive pressure decreases at this time and if the vehicle has been driven with a large motor displacement, the motor displacement is reduced, whereas if the vehicle has been driven with the minimum displacement, the minimum motor displacement is sustained. As a result, the quantity of oil intake required to rotate the traveling motor decreases to inhibit the occurrence of cavitation.
However, if a reverse operation is performed at the accelerator pedal while the vehicle is traveling, i.e., if the rear side of the accelerator pedal is depressed while the vehicle is traveling forward, the control valve is switched to the reverse travel position and, as a result, the motor drive pressure increases in a state in which the vehicle is traveling with an inertial force. Consequently, the motor displacement increases to lead to an increase in the quantity of oil intake required for traveling motor rotation, giving rise to the risk of cavitation.